1. Department of Electrical Engineering and Computer Science; Massachusetts Institute of Technology; Cambridge MA 02139 USA
2. Department of Mechanical Engineering; Massachusetts Institute of Technology; Cambridge MA 02139 USA
3. Department of Energy Engineering; Gyeongnam National University of Science and Technology; Jinju 52725 South Korea
4. College of Engineering and Applied Sciences; Nanjing University; Nanjing Jiangsu 210093 P. R. China